High-Performance Magnetic-core Coils for Targeted Rodent Brain Stimulations

Abstract

Objective and Impact Statement. There is a need to develop rodent coils capable of targeted brain stimulation for treating neuropsychiatric disorders and understanding brain mechanisms. We describe a novel rodent coil design to improve the focality for targeted stimulations in small rodent brains. Introduction. Transcranial magnetic stimulation (TMS) is becoming increasingly important for treating neuropsychiatric disorders and understanding brain mechanisms. Preclinical studies permit invasive manipulations and are essential for the mechanistic understanding of TMS effects and explorations of therapeutic outcomes in disease models. However, existing TMS tools lack focality for targeted stimulations. Notably, there has been limited fundamental research on developing coils capable of focal stimulation at deep brain regions on small animals like rodents. Methods. In this study, ferromagnetic cores are added to a novel angle-tuned coil design to enhance the coil performance regarding penetration depth and focality. Numerical simulations and experimental electric field measurements were conducted to optimize the coil design. Results. The proposed coil system demonstrated a significantly smaller stimulation spot size and enhanced electric field decay rate in comparison to existing coils. Adding the ferromagnetic core reduces the energy requirements up to 60% for rodent brain stimulation. The simulated results are validated with experimental measurements and demonstration of suprathreshold rodent limb excitation through targeted motor cortex activation. Conclusion. The newly developed coils are suitable tools for focal stimulations of the rodent brain due to their smaller stimulation spot size and improved electric field decay rate.

A C-shaped miniaturized coil for transcranial magnetic stimulation in rodents

“…One main challenge of designing miniaturized coils was caused by the trade-off between coil diameter and E-field strength [76]. To make the coil size suitable for rodent, extremely high current was required to achieve the same stimulation strength as in human TMS coil [21,34], which might cause excessive heating, electromagnetic stress, and other technical difficulties. Given this physical constraint, further improvements could still be made to the current design.…”

“…However, the figure-eight coil cannot be easily combined with standard electrophysiology since it is difficult to implant a straight penetrating electrode under the center of the coil (where maximum stimulation intensity exists) while keeping the coil close to the brain surface (figure 1(B)). Other alternative designs [31][32][33][34] improved the stimulation focality using analytical models but at the same time generated E-field distributions different from those of circular or figure-eight coils, which are most commonly used in humans. Consequently, the neural responses elicited with these coils cannot be directly compared with those in human TMS studies.…”